Haemophilus ducreyi causes chancroid, which facilitates HIV transmission. In an experimental infection model in humans, papules develop within 24 h and either resolve or evolve into pustules. Pustules contain an abscess composed of PMNs and macrophages, while T cells, myeloid dendritic cells (DC) and macrophages form a loose granuloma below the abscess. In the failed pustular state, H. ducreyi is surrounded by phagocytes that do not ingest the organism. Although pustules form in some subjects, all infected sites resolve in other subjects. When pustule formers and resolvers are re-infected, they tend to segregate towards their initial outcome, confirming a host effect. PMNs and macrophages isolated from the blood of those who formed pustules twice (PP group) or resolved twice (RR group) did not differ in their ability to ingest H. ducreyi, suggesting that the environment at the site of infection modulates phagocytosis. In PP and RR subjects infected a third time, lesional transcripts differed between the groups, confriming that their local immune responses are different. Myeloid DC from both groups had a common transcript response to H. ducreyi, but each group had unique responses. Infected PP DC upregulated transcripts that were markers of DC maturation and markers of semi-mature or regulatory DC and downregulated transcripts known to promote DC maturation or antigen processing. Infected RR DC only upregulated transcripts of DC maturation. Our overall hypothesis is that the interaction of H. ducreyi with DC, and the interaction of infected DC with T cells are key determinants of effective (RR) or ineffective (PP) phagocytic responses in lesions. DC from the PP group likely promote a dysregulated Type 1 and Tr response that leads to an antiphagocytic cytokine environment, while DC from the RR group promote a Type 1 response that leads to a pro-phagocytic environment. To test these hypotheses, our specific aims include: comparison of transcripts in lesions collected 48 h after a third infection of the RR and PP groups; comparison of the gene expression and proteomic profiles of DC derived from the PP and the RR groups exposed to live H. ducreyi; testing whether DC pulsed with live H. ducreyi and co-cultured with T cells from the RR group result in Type 1 responses while co-cultures derived from the PP group lead to Type 1 and Tr responses; examination of whether the cytokines generated by H. ducreyi - DC - T cell interaction promotes or inhibits phagocytosis of the organism and of the mechanisms underlying the modulation of phagocytosis. We will confirm our in vitro results with observations made on biopsies obtained from PP and RR subjects who are re-infected. Lay summary: H. ducreyi is a germ that causes genital ulcers. When we infect human volunteers on the arm with the germ, some people develop disease while others clear the infection. We seek to answer an important question: Why do some people who become infected with a germ get sick, while others do not?